Programmable impedance output driver

摘要

An output driver circuit is disclosed that generates an accurate and predictable output impedance driver value corresponding to a programmable external impedance. The output driver circuit includes an external resistance device, voltage comparator device, control logic, an evaluate circuit and off-chip driver (OCD) circuit. Voltage from the external resistance device (VZQ) is compared with voltage created from the evaluate circuit (VEVAL) by the voltage comparator device, which indicates to the control logic whether VEVAL is greater than or less than VZQ. The control logic will adjust the evaluate circuit accordingly with a count until the two voltages are basically equal (i.e., the count is alternating between two adjacent binary count values). At which time the control logic operates the OCD with the lower of the two adjacent count values to produce a proper and predictable driving impedance.